The present invention relates to a process for the production of an extra fine cobalt powder, whereby a finely-divided, uniformly-grained and carbon-free cobalt powder can be produced for powder metallurgical purposes.
In powder metallurgy, especially in the hard metal industry, precise requirements are set for the cobalt powder used. The cobalt powder used by he hard metal industry must be finely-divided, with an average grain size of 1-2 .mu.m and with a specific surface area of over 10,000 cm.sup.2 /g. Impurities C, Fe, Mn, S, Na, Ca and Mg must not exceed 0.03%. In addition it is required that the cobalt powder have a certain crystal form, grindability and activity in sintering. Its oxygen content must be constant, in general less than 0.5%.
So far the most commonly used process for the production of the extra fine cobalt powder used by the hard metal industry is to decompose cobalt oxalate in a hydrogen atmosphere at about 500.degree. C. The initial material in the process is a cobalt chloride solution, from which cobalt oxalate is precipitated by means of either oxalic acid or ammonium oxalate.
Cobalt oxalate is separated by filtering, washed several times with water, dried, and pyrolysed in a reducing atmosphere at about 500.degree. C. The product obtained is a cobalt powder with the desired grain size. The chemical impurities originate in the raw materials used. The carbon content for its part depends on the degree of decomposition of the oxalate. The grain growth and sintering of the cobalt powder set the limits for the maximum temperature and period which can be used. This production process has been used for decades, and its various stages are well known. The greatest weakness of the process is its high cost and the inevitable inclusion of a certain proportion of carbon in the cobalt.
Processes have been developed in which part or all of the oxalic acid is replaced by a less expensive reagent, such as carbonate. A complete carbonate precipitation demands, however, elevated pressure and temperature, i.e., an autoclave. If, on the other hand, only part of the oxalate is replaced by carbonate, it is not possible to produce a homogenous extra fine cobalt powder. In the carbonate decomposition the carbon content will be of the same order as in the oxalate process.
Carbon-free, finely-divided cobalt powder can be produced by pyrolysing either cobalt chloride directly or by reducing cobalt oxide made from cobalt chloride. When impure Ni- and Na-bearing cobalt chloride solution is used as the raw material, it is easier to separate the nickel by liquid-liquid extraction. The sodium can be washed with water from the cobalt oxide, which is the intermediate product. Pure cobalt chloride can naturally also be produced by leaching cobalt in hydrochloric acid, as in the oxalate process.
When fine cobalt powder is produced by the oxalate, carbonate or chloride process, the processes are multi-stage ones with both hydrometallurgical and pyrometallurgical operations.